Motor control system



Oct. 6, 1942. D. Q WR|GHT MOTOR CONTROL SYSTEM Filed May 20, 1941 HOISTLOWER 7o 4 3 2 l I INVENTOR. DAVID C. WRIGHT I20 I60 200 PER CENT'TORQLE 4 .PZMU Ema AT TOR NEYS Patented (let. E, 194 22 antic MoronCONTROL SYSTEM David C. Wright, Euclid, Ohio, assig'nor to The ElectricGcntrcller & Manufacturing Company, Cleveland, Ohio, a corporation ofOhio Application May 20, 1941, Serial No. 394,321

t Claims.

usual. shunt or, separately excited motors, when energized from aconstant voltage source, have disadvantages during hoisting operationsof industrial cranehoists. For example, when a shunt or separatelyexcited motor is energized from a constant voltage generator and used inhoisting service, the hoisting speeds as well as the lowering speeds areapproximately constant for any given setting of the speed control deviceregardless of the size of the load. However, for

hoisting loads it is undesirable'to have constant speed characteristics.In fact, a hoisting speed inversely proportional to the size of the loadis desired. Therefore, these types of motors have not been used to agreat extent for' operating industrial crane hoists. Y

The well-known Ward Leonard system, using separately excited motors, hasbeen used for hoisting and lowering of power-shovels and the like,wherein little or no control of lowering speeds is required. In a WardLeonard system for such purposes the generator has a separately excitedshunt field and a self-excited differentially connected series'neld andthe motor has a separately excited shunt field. A direct currentgenerator of this type, when driven at a uniform speed and connected tosupply power to the separately excited motor, causes the motor to havedesirable speed-torque characteristics for hoisting loads which can bemade to approach those of a direct current series motor connected to afixed voltage generator. The Ward Leonard system, in spite of itsexcellent hoisting characteristics, has not been used generally forindustrial crane hoists because of its uncontrollable loweringcharacteristics. For example, the speed-torque characteristics of themotor of such a. Ward Leonard system are not suitable when heavyoverhauling loads must be lowered slowly and at controlled speeds.

When lowering overhauling loads, the motor must supply a retardingtorque and to do so it operates as a generator tending to generate ahigher voltage than the variable voltage generator to which its armatureis electrically connected. The voltage of the separately excited motorwhen acting as a generator in retarding the load opposes the voltage ofthe variable voltage generator and the current generated by the motortends to reverse the current through the series field of the generator.This weakens the differential effect of the two generator fields andcauses the generator to develop a continuously cumulative highervoltage. Consequently, the motor must increase its voltage in order tocontinue to generate a voltage greater than the voltage of the generatorand tends to run at excessive speeds and does not retard the overhaulingload sufllciently. Therefore, although the differential field of avariable voltage generator improves the speed-torque characteristics in.hoisting, it renders the lowering characteristics of the separatelyexcited motors unstable and consequently such generator and motorcombinations have not been used in crane hoisting service where accuratespeed control in lowering is essential.

Th principal object of the present invention is to provide a control fora Ward Leonard system by which desirable lowering characteristics areobtained while retaining all of its desirable hoisting characteristics.

Another object is to provide a control system for a separately excitedshunt motor so that it has satisfactory speed-torque characteristicsboth in lowering and hoisting loads.

A more specific object is to provide a system for controlling theoperation of a variable voltage generator and separately excited motorcombination for use in hoisting service in which desirable speed-torquecharacteristics are obtained both for lowering and hoisting.

A correlative object is to provide a control system for a variablevoltage generator and sepa- 1 rately excited motor in which the motormay safely lower overhauling loads at controlled speeds.

Another object is to provide a Ward Leonard system employing avariaolevoltage generator having a separately excited field winding and adifferentially connected series field winding in which a. separatelyexcited shunt motor energized by the generator has suitable hoisting andlowering characteristics.

Other objects and advantages will become apparent from the followingspecification wherein reference is made to the drawing, in which:

Fig. l is a. complete wiring diagram illustrating a preferred embodimentof the present inven tion, and

Fig. 2 is a graph illustrating the speed-torque characteristics of theseparately excited shunt motor of the invention.

In accordance with this invention, a variable voltage generator having aseparately excited field winding and a differentially connected seriesfield winding is connected to supply power to a separately excited hoistmotor. A normallyclosed switch is connected across the difierentiaL- 1yconnected series field winding. During hoisting, the normally closedswitch is open and the generator has the common drooping voltagecurrentcharacteristics. When lowering, the spring-closed switch is closed andthe series field is disconnected causing the generator to have shuntcharacteristics. The strength of the separately excited field may thenbe varied and several different lowering speeds obtained which areapproximately constant regardless of the size of the load.

Direct current is supplied to the control system from a suitable source(not shown) through power conductors to and it, one of which, shown asthe conductor Ii, is grounded at i2.

An alternating current motor i is connected for energization from asource of alternating current (not shown) through the conductors Li, L2and L3. The motor I5 is arranged to drive a direct current generator 20at a substantially constant speed through a shaft I3. Any other suitablemeans may be used to drive the generator 2c. The generator 20 has anarmature winding 28, a separately excited field winding 22, and adifierentiaily connected series field winding 23. The armature winding2! of the generator 2i is adapted to be connected through the conductors21 and 28 to an armature winding 2% of a direct currentmotor 25 having aseparately excited field winding 26. The motor 25 is drivingly connectedto a hoisting drum 39 on which a cable is wound from which is suspendedaload-3l. A spring-applied, electromagnetically-released friction brake32 is provided for'the motor 25.

terposed in the conductor 28 are normally open contacts 35a of anelectromagnetic contactor 35 which has an operating winding 3510 andnormally closed contacts 85!). The contacts 35b normally complete ashunt circuit around the series field winding 28 and the normally opencontacts 35a through a conductor 29. Also interposed in the conductor 28are normally open contacts tfia. of an electromagnetic contactor 88which has an operating winding Sew and normally closed contacts 36?).The contacts 86b normally complete a dynamic braking circuit across thearmature winding 24 of the motor 25 through a dynamic. braking resistor31. The brake 32 is normally applied by a spring 33 and is released uponenergization of an operating winding 32w.

Direct current for the separately excited field winding 25 of themotorfit and the separately excited field winding 22 of the generator tois obtained from the conductors it and it through a knife switch 38. Theswitch 38 when closed connects the conductors id and i! to theconductors so and M respectively. .The field winding 28 is connectedacross the conductors to and M through the conductors 52 and d8. Inseries with the conductor 33 is an operating winding 4510 of anelectromagnetic relay 55 having normally open contacts the.

Electromagnetic contactors ts and fill are arranged to control thepolarity of the separately excited field winding 22. The contactor athas an operating winding 36m and normally open contacts don. and'ttb.The contactor M has an operating winding dim and normally open con tactsdie and Mb. The contactor 66 when closed connects the conductors Al and40 to the conductors 5| and 50 respectively, and the contactor B! whenclosed connects the conductors 4| and 40 to the conductors 5 8 and 5!respectively. The conductors 5i] and 5! are connected to the terminalsof the field winding 22 and a resistor 53 is interposed in the conductor59.

The resistor 53 is divided into sections in Similarly, sections of theresistor 53 of the same or difierent ohmic value than those controlledby the contactors 55, 56 and 51 are arranged to be successively shortcircuited in the hoisting direction of operation by means ofelectromagnetic contactors 65, 66 and 67. The contactors 65, and 61 aresimilar to the contactors 55, 56 and 51 respectively. The contactor 55has an operating winding 65w and normally open con,- tacts 65a and 651);the contactor 56 has an operating winding 66w and normally open contacts560. and 66b; and the contactor '6'! has an operating winding 6110 andnormally open contacts 51a. The ohmic resistance of the respectivesections of the resistor 53 which each of the contactors 55, 56, 51, 65,66, and E1 is adapted to short circuit is independently adjustable asindicated.

To selectively control the .operation of the various electromagneticcontactors a multi-posi-' tions, are movable into engagement withcontact segments 12 to 15 inclusive and, some of which, in the hoistingpositions, are movable into engagement with contact segments W to diinclusive. In addition, a contact segment 82 engages one of the circuitterminals ii, in all positions. All or" the contact segments 12 to 82inclusive are electrically interconnected.

A more complete understanding of the control system of this inventionmay be had from a consideration of the following description of itsoperation in which it is assumed that the motor 15 is driving thegenerator 20 at a substantially constant speed. Closure of the switch 38causes energization of the field winding 26 of the motor 25 bycompleting a circuit from the conductors it and it through theconductors 4d and M, and th conductors s2 and 43. Closure of the switch38 also connects the contact segment as of the master switch ii) to theungrounded conductor id through the conductors 40 and 8d and one, of thecircuit terminals H.

Movement of the master switch it in the hoisting directionresults inenergization of the field winding 22 of the generator is at a polaritysuch as to cause its voltage to drive the motor 25 int.

dta and deb of the contactor at due to the energization of its operatingwinding tw. The winding 36w is energized over a circuit extending fromthe conductor H3 through the switch 38, the conductors so and at, one ofthe circuit terminals H, the contact segment 82, the contact segmentaccesses 78, another of th circuit terminals ii, a conductor 35, and thewinding 46w to ground, In order to make the drawing as clear aspossible, one terminal of each of the contactor operating windings andthe brake operating winding is shown connected to ground as is theconductor H Energization oi the winding 32w causes corn-- pression ofthe spring and release or" brake Elosure of the contacts llis andmovement the master switch from the off position in the r direction alsocompletes an energizing 301 the winding 36w of the contactor segment llthrough the conductor .tacts 3 23a, a conductor and the winding Thecontactor in response to rgizaticn of its winding 38w opens its cono tointerrupt the dynamic braking cirrough the resistor 37 and closes itscon i can to partially complete a circuit through conductor 25; betweenhe generator 525) and otor L o ement of the master switch in thehoisting direction also completes an energizing circuit for riding 3510of the contactor from the con v segment 58 through a conductor and theto ground. The contactor in re "e to the energization of its operatingwind-- closes its contacts 35d and opens its con o. Closure of thecontacts 35a completes ctor-generator circuit "through t1 e cliiferco,.nected series field winding 23 and the and opening of the contacts3% "roots the shunt circuit around the series vinding through theconductor 29. h the master switch it in the first hoisting ition all ofthe resistance 53 is eiiective to maintain the strentgh of the fieldwinding 22 at a low value and the motor 25 operates to hoist t e load tialong a speed-torque characteristic such as the curve Hill of Fig. 2'.It will be seen load Si is large, the hoisting speed is e1 slow, whereasif the load is small, the hoisting speed is somewhat increased. Thedrooping nature of the curve I63 is due to the eiiect'or the seriesfield winding 23. The hoisting torque of the motor 25 may be incr asedabove that obtainable in the first position moving the master switch tothe second, or fourth hoisting positions, which causes selectiveoperation of the contactors 65, 86, and Movement of the master switch 70to the second hoistin position causes the short circuit oi a section ofthe resistor 53 by the contactor $5, movement to the third hoistingposition causes short circuiting of an additional resistance ection bythe contactor 6B, and'movement to rth hoisting position causes shortcircuit ng of another section of the resistor 53.. Each time aresistance section is short circuited the excitation of the fieldwinding 22 is increased and consequently the voltage of the generator isincreased. The energizing circuit for the winding 85w to ground.

of the contactor G5 is completed in the [5 second, third, and fourthhoisting positions from the contact segment l9 through a conductor 99;

the energizing c suit for the winding 5820 of the contactor is completedin the third and fourth hoisting positions from the contact segment Bilthrough a conductor 93 and the now closed contacts and the energizingcircuit for the winding Slw is completed in the fourth hoisting positionfrom the contact segment ll through a conductor and the now closedcontacts 55b. in order to simpiiiy the disclosure, a simple interlocircontrol for the field controllih contactors has been It will heunderstood by those skilled in the art that suitable time delay devicesmay be incorporated in the circuit to delay the successive resistanceshunting operations of the contactors.

Closure of the contactor $5 the field strength of the generator so themotor operates act ric such as the curve t e con tactors and 0th coperates along a curve such a c three of the contactors fill a closedthe field strength of the gene" maximum the motor a curve such as Thedroq speed torque characteristics it is due to th differential effect0.- ser es field lis speed of the I i i ite limits he "i e operator andthat he loa iii is extremely s n hoisting speeds -n excess oi "uli loadose are obtainable, whereas the r large only slow h isting speeds Thistype of speed-toroue shar found to he the most desire to the currentvoltage gen a gene tor such as the had its onferentieily connectedseries winding :23 connected the motor-generator circuit when its separae excited 2'2 energized at such "c ty as to cause the motor to rotate ina direction to lower the load excessive lowering speeds would result theload 3i were enough to overhaul the motor 25 and the associated gearingand is because when the motor is overhauled it tends to operate as agenerator and supply voltage to the generator and drivethe generator 20as a motor. Since the series field winding 23 is con necteddifferentially with respect to the separately excited field winding whencurrent from the motor 25 flows through the f ld the field so isreversed and instead of to rig differential with respect to the field 22cumulative. Cumulative action of the two fields 22 and 23 causes anincrease in the voltage of the generstar as. This has the immediateeffect of causing the motor 25 to increase its speed in an attempttogencrate a voltage greater than that of the generator to.Consequently, during lowering, the motor tends to run at excessivespeeds when subjected to overhauling loads if the series field 23 ispermitted to he energized.

In accordance with the present invention the foregoing lowering defectsare eliminated and desirable lowering characteristics are obtained whileretaining the desirable hoisting characteristics. When the master switchis moved to the first position in the lowering direction the brake 32 isreleased and the contactor 35 energized as in hoisting except that' thecontact segment l2 completes the energizing circuit to-the field winding22 is now of such polaritythat they voltage of the generator 26 causesthe motor 25 to operate to lower the load 3|.

A circuit is not completed to the winding 35w of the contactor 35 whenthe master switch 16 is moved to the lowering positions and consequentlythe contacts 351: maintain the series field winding 23 unenergized andthe normally closed contacts 352) complete the generator-motor circuitthrough the conductor 29 around the field winding 23; As a result thespeed-torque characteristic of the motor 25 in the first loweringposition is a curve such as the curve Hit of Fig. 2. Curve N16 issubstantially a straight line and is nearly parallel with the torqueaxis. A substantially constant lowering speed is obtained regardless ofthe weight of the load, although extremely heavy loads are loweredsomewhat. faster than lighter loads.- If the load 3| is overhauling,

the motor 25 becomes a generator and, drives-the generator 20 as amotor. Since the series field winding 23 is not in the circuit, thevoltage of the generator 2|! is predetermined depending upon theposition of the master switch 10, and as-son as the voltage of the motor25 exceeds that of the generator 2|! a braking action takes place toslow down the lowering speed of the load 3|.

Increased lowering speeds for both power low- I ering. and regenerativebraking lowering canbe obtained by moving the master switch ill tothesecond, third, or fourth position in the lowering direction. In thesecond lowering position a circuit is completed from the contact segmentit through aconductor 93 to the winding 5520' of the contactor 55. As aresult of the energization of the winding 55w the contactor 55 closesits contacts 55a to short circuit a section of the resistor 53, thusincreasing the voltage applied to the field winding 22 and causing themotor to operate at increased speed along the curve Hi1, Closure ofthecontacts 55b completes a circuit from the circuit terminal llengageable by the contact segment through the conductor 96 to thewinding 56d: of the contactor 56, and if the 7 master switch I0 is nowmoved to the third position in the lowering direction or if it isalready in the third position, the contactor 56 closes to short circuitanother section of the resistor 53. The motor now lowers the load 3|along the curve m8 of Fig. 2.

A further increase in lowering speed may be obtained by moving themaster switch to the forth lowering position completing a circuit fromthe contact segment 15 through a conductor 95 and the now closedcontacts 55b to the winding ilw. As a result of the energization of thewinding 5111;, the contacts 5111 close to short circuit a furthersection of the resistor 52 and apply full voltage to the field winding22. The speed-torque characteristic of the motor 25 is now shown by thecurve I09.

The operator thus has complete control of the whether theloadisoverhauling or not. The

motor 25 safely lowers overhauling loads at controlled speeds.

If at any time during movement of the load 3| the shunt field 26 of themotor 25 is reduced in strength, the winding d5w permits drop out o!the'relay 45 and consequent opening of the contacts 45a. Opening of thecontactsd5'a deenergizes the winding 32w of the brake 32 which isthereupon applied by the spring 33 to stop the motor 25. Concurrently,the winding the of the contactor 36 is deenergized and the contactor 35operates to interrupt the motor-generator circuit by opening thecontacts 86a and to close the dynamic braking circuit through theresistor 31 by closing the contacts 3%.

Movement of the master switch it to the off position also interrupts thecircuits'to the windings 32w and 36w to slow down and stop the motor 35by a combination of friction and dynamic braking I claim:

1. In a motor control system, the combination with a differentiallycompound generator and a separately excited shunt motor connected in aloop circuit and means to permit regenerative braking of said motorthrough said generator, of a means to render the series field ofthegenerator ineffective to change substantially the total magnetic fiux ofthe generator during said braking while maintaining the shunt field ofthe motor lowering speed of the motor 25 regardless of efiectlve foruseful braking. r

2. In a motor control system in which a shunt motor is subject tooverhauling loads, the combination with said motor, a generatorconnectedin a closed circuit with said motor, said generator having aseparately excited field winding and a differentially connected seriesfield winding, and means for selectively varying the polarity of saidseparately'excited field winding of the generator toreverse the torqueof said motor, selectively, of control means operable for rendering saidseries field operative and inoperative selectively, whereby the speed ofsaid motor is controllable when it is being rotated by the overhaulingload.

3. In a system of motor control for raising and lowering a load, thecombination with a separately excited motor drivingly connected to saidload,

a generator connected in a closed circuit with said motor, saidgenerator having a separately excited field winding and a differentiallyconnected series field winding, and means for selectively varying thepolarity of said separately excited field winding of the generator tocause said motor toselectively raise and lower said load, of meansoperable for rendering said series field operative and inoperativeselectively, whereby said motor is controllable while both hoisting andlowering said load.

4. A motor control system according to claim 2 characterized in thatsaid separately excited field winding is controlled by a master switchand said control means is operable to render the series fieldinoperative by movement-of said master switch to positions to reversethe polarity of the by, said generator having a shunt field winding anda difierentlally connected series field winding and said motor having ashunt field winding, and means for controlling the polarity of one ofsaid shunt field windings for efiecting hoisting and lowering operationsby said motor, of switching means for rendering the generator seriesfield winding efi'ective during hoisting operations and for renderingsaid generator series field winding inefiective during loweringoperations.

7. In a motor control system, the combination with a differentiallycompound generator and a shunt motor connected in a loop circuit andmeans to permit regenerative braking 01' said motor through saidgenerator, of ameans to render the series field of the'generatorineftective to change substantially the total mag- DAVID C. WRIGHT.

